1. Field of the Invention
This invention relates to a control circuit for controlling a transfer device that transfers materials or works to be pressed to and from the predetermined positions in a press mold.
2. Description of the Prior Art
Transfer devices of constructions operable in 2-dimensional and 3-dimensional manners have been widely known. The former type shifts (advances and retracts) a pair of feed bars 1 in the direction of X-axis for a distance corresponding to a stroke as shown in FIG. 1, and also in the direction of Y-axis for a predetermined distance of a stroke for clamping and unclamping the materials or works. The latter type, on the other hand, besides of the above described operations, shifts (up and down) the feed bars 1 in the direction of Z-axis as shown in FIG. 2.
Heretofore the above described operations of the transfer device have been ordinarily carried out utilizing a power supplied from a crank shaft of, for instance, a press through a cam mechanism. However, recently another arrangement in which a press-stroke angle (crank angle of the shaft) is detected by an encoder, and the above described movements of the feed bars are caused by separately provided servo-actuators in response to the detected press-stroke angle, has been also proposed.
In the afore-mentioned 2-dimensional transfer device, the material feeding operation and the clamping and releasing operation are ordinarily carried out at a timing as shown in FIG. 3. During such operations, a time interval corresponding to 120.degree. crank angle (1/3 of the press stroke) measured from the completion of opening (at 120.degree. crank angle) of fingers 2 (see FIGS. 1 and 2) to start-closing instant (at 240.degree. crank angle) of the same is utilized for the effective pressing operation of the press. Accordingly, in the transfer device where the work is supplied in synchronism with the movement of the press, the depth formed by press (drawing) in the products is less than 1/3 of the press stroke. If it is desired to utilize 1/2 of the press stroke for the press operation, a time interval corresponding to such an amount of press stroke angle must be assigned between the opening completion instant and the start-closing instant of the fingers 2 during substantially returning stroke of the feed bars 1. This inevitably delays the returning stroke of the feed bars 1, and hence reduces the press speed.